Optical data transmission system employing polarization-shift, multiple-cavity laser

ABSTRACT

An internally-modulated laser in which first, second and third reflectors and a polarization means are oriented with respect to each other both to provide a first optical resonant cavity, defined by the first and second reflectors, and a second optical resonant cavity, defined by the first and third reflectors. A laser medium is located in the common portion of the cavities between the first reflector and the polarization means, and an individual data-signal-responsive modulator is situated in each respective individual portion of the cavities between the polarization means and the respective second or third reflector. The first cavity supports a given polarization and the second cavity supports a polarization orthogonal to the given polarization. The modulated output wave energy provided by this apparatus includes two separate orthogonally polarized data channels.

Unite States Patent 91 Herzog et al.

[ Oct. 16, 1973 [75] Inventors: Donald George Herzog,

Collingswood Charles William [52] US. Cl. 250/199, 331/945 [51] Int. Cl.H04b 9/00 [58] Field of Search 250/199; 331/945 C,

331/945 S; 307/883; 321/69 R, 69 NL; 332/751 [5 6] References CitedUNITED STATES PATENTS 3,482,184 12/1969 Schneider 33l/94.5 C 3,551,84012/1970 Crowell 3,423,588 l/l969 Uchida 3,663,897 5/1972 Broom 331/945 COTHER PUBLICATIONS J. Kupka, Electronics Letters, Freq. Mod. SingleModes, Vol. 2/7 No. 2, l/26/68 Primary ExaminerAlbert J. MayerAttorney-Edward J. Norton [57] ABSTRACT An internally-modulated laser inwhich first, second and third reflectors and a polarization means areoriented with respect to each other both to provide a first opticalresonant cavity, defined by the first and second reflectors, and asecond optical resonant cavity, defined by the first and thirdreflectors. A laser medium is located in the common portion of thecavities between the first reflector and the polarization means, and anindividual data-signal-responsive modulator is situated in eachrespective individual portion of the cavities between the polarizationmeans and the respective second or third reflector. The first cavitysupports a given polarization and the second cavity supports apolarization orthogonal to the given polarization. The modulated outputwave energy provided by this apparatus includes two separateorthogonally polarized data channels.

5 Claims, 5 Drawing Figures B GEN L SYNCH A il" 308 STREAM P2 R SOURCEPUMP 2 l l l P|+ P2\+ l P LASER it- MEDIUM P? I I; P MODUIiATO RPATENTEUucI 16 ms 3.766; 393

SHEET 10F 2 POLARIZATION MODULATION REFLECTOR PUMP m8 MEANS MEANS noREFLECTOR lo iw P l I i LASING :1 MODE- POLARIZAT|0N K mzmum LOCKINGSHIFTER T z l '00 CIRCULATING 'I/GUTPUT "QM I' Q' 2 v g PERIODIC DATAENERGY SIGNAL BIT GEN STREAM 2-1- Ne SOURCE SYNCH MODULATION MEANS H00 1l I' 1 ELECTRO- I OPTICAL '22 1 CRYSTAL l L 1 I24 \l26 0.0. BIAS 1SIGNAL a. I COUPLER 2 FROM PERIODIC FROM DATA SIGNAL GEN STREAM SOURCEH8 A 1- J H H [L CIRCULATING WAVE v 7 ENERGYP| OUTPUT WAVE ENERGY in nP2 2.2. 200 200 DATA BIT STREAM OPTICAL DATA TRANSMISSION SYSTEMEMPLOYING POLARIZATION-SHIFT, MULTIPLE-CAVITY LASER This inventionrelates to optical data transmission systems incorporating aninternally-modulated laser and, more particularly, to improved apparatustherefor capable of transmitting data at a high effective informationrate with relatively high power efficiency.

Briefly in accordance with one aspect of the present invention, first,second and third reflectors and a polarization means are oriented withrespect to each other both to provide a first optical resonant cavity,defined by the first and second reflectors, for wave energy of a givenwave length which has a first given linear polarization, and to providea second optical resonant cavity, defined by the first and thirdreflectors, for wave energy of this given wavelength having a secondlinear polarization orthogonal to the given linear polarization.Therefore, the two optical resonant cavities include one portion whichis common to both cavities and other respective portions which areindividual to each of the respective cavities. A lasing medium capableof generating wave energy at the given wavelength is located in thecommon portion and an individual dataresponsive polarization modulatoris situated in each respective individual portion of the cavities. Theoutput wave energy provided by this apparatus includes two separate,orthogonally-polarized data channels.

This and other features and advantages of the present invention willbecome apparent from the following detailed description, taken togetherwith the accompanying drawing, in which:

F IG. 1 is a block diagram of an optical data transmission systememploying only a single optical resonant cavity incorporating aninternally-modulated modelocked laser for transmitting digital data;

FIG. 1a shows an alternative embodiment of the modulation means of FIG.1;

FIG. 2 is a timing diagram useful in explaining the operation of thearrangement shown in FIG. 1;

FIG. 3 illustrates a first embodiment of the present inventionparticularly suitable for transmitting digital data, and,

FIG. 4 illustrates a second embodiment of the present inventionparticularly suitable for transmitting analog data.

Referring to FIG. 1, there is shown a pair of spaced, parallelreflectors 100 and 102 defining an optical resonant cavity of givenlength therebetween. Within this optical resonant cavity, and in spacedrelationship with both reflectors 100 and 102, is polarization means104. Polarization means 104, by way of example, may consist of a Glanprism or a Wollaston prism.

Lasing medium 106 is situated, as shown, Within the optical resonantcavity between and in spaced relationship with reflector 100 andpolarization means 104. Lasing medium 106 may consist of neodymium dopedYAG, by way of example. Lasing medium 106 is excited by incidentradiation (designated by the arrows) from pump 108.

Situated within the optical resonant cavity, between reflector 102 andpolarization means 104 and in spaced relationship therewith, ismodulation means 110. The embodiment of modulation means 110 shown inFIG. 1 is composed of a separate mode-locking element 112 and apolarization shifter 114. (An alternative modulation means 110a, shownin FIG. 1a, will be discussed below.)

Mode-locking element 112 may consist of means, such as an electro-opticcrystal, for phase modulating wave energy passing therethrough inaccordance with the signal applied thereto from periodic signalgenerator 116, or, alternatively, it may consist of means, such as anacousto-optic element or a signal-responsive absorbing element, foramplitude modulating the wave energy passing therethrough in accordancewith the signal applied thereto from periodic signal generator 116.

Polarization shifter 1 14, which also may consist of an electro-opticcrystal, shifts the polarization of wave energy passing therethrough inaccordance with the signal level applied thereto from data bit streamsource 118. The data bit stream of source 118 is synchronized by asynchronizing input applied thereto from periodic signal generator 116,as shown.

The signal from periodic signal generator 116 is preferably sinusoidal,but may have a different wave shape. In any case, as indicated in block116 of FIG. 1, the frequency of the periodic signal is equal to theproduct of any given integer, N, and the speed of light, c, divided bytwice the optical length, l, of the optical resonant cavity defined byreflectors and 102. Data stream source 118 applies a first signal levelto polarization shifter 114 in response to data bits manifesting abinary ZERO and a second level to polarization shifter 114 in responseto data bits manifesting a binary ONE."

The operation of the apparatus shown in FIG. 1 will now be discussed. Inresponse to the pumping thereof by pump 108, lasing medium 106 iscapable of emitting wave energy at a given wavelength. Any of this waveenergy which is incident on either reflector 100 or reflector 102,respectively, will be reflected therefrom and retained within theoptical resonant cavity defined by these reflectors. I

Polarization means 104 is oriented to pass there.- through and retainwithin the optical resonant cavity only that first component thereofwhich has a given linear polarization P, (polarized, say, in the planeof the paper). Any second component of wave energy having a linearpolarization P orthogonal to the given polarization (polarized, say,perpendicular to the plane of the paper) incident on the left side ofpolarization means 104, would he, were it present, directed through thetop of polarization means 104 to the outside of the optical resonantcavity defined by reflectors 100 and 102. Similarly, any of this Plinearly polarized second component of wave energy incident on the rightside of polarization means 104, which may be present, is directedthrough the bottom side of polarization means 104 to the outside of theoptical resonantcavity' defined by reflectors 100 and 102, andconstitutes output wave energy. Thus, the presence of polarization means104 within the optical resonant cavity defined by re-1 flectors 100 and102 ensures that the circulating wave.

energy retained within the optical resonant cavity and incident on theleft side of polarization means 104 must have only the given linearpolarization P,. However,

due to the presence and relative location of'polarization shifter 114,the wave energy incident on the right side of polarization means 104 mayinclude a P linearly polarized second component.

As stated above, polarization shifter 114 may be an electro-opticcrystal. As is known, an electro-optic crystal, in general, produceseliptically polarized output wave energy in response to incident waveenergy, regardless of whether the incident wave energy is linearlypolarized or not. Any such eliptically polarized wave energy can beconsidered to be the resultant of a first component having the givenpolarization P, and a second component having a polarization Porthogonal thereto. As is known, an electro-optic crystal operates bydividing the incident wave energy into respective ordinary andextraordinary wave components, which are oriented in quadrature withrespect to each other, and then provides a relative difference in phasedelay between the ordinary and extraordinary wave compo nents. For anygiven electro-optic crystal of any given path length, this relativedifference in phase delay is a function of the magnitude of the signallevel applied to the electro-optic crystal.

The sensitivity of polarization shifter 114 (i.e., the change inpolarization per change in level of the data signal applied thereto)depends upon the effective optical length of the polarization shifterelectro-optic crystal and the ratio of the peak amplitudes of theordinary and extraordinary waves traveling therein. The effective lengthof polarization shifter 114 is twice its actual length, since due to thepresence of reflector 102, wave energy traveling both from left to rightand from right to the left passes through polarization shifter 114,thereby doubling its sensitivity. Further, the electrooptic crystal ofpolarization shifter 114 is preferably oriented with its optic axislying in a plane normal to the direction of travel of the wave energytherethrough and being disposed at an angle of substantially 45 withrespect to each of respective linear polarizations P, and P Under theseconditions, the relative amplitudes of the ordinary and extraordinarywave components will be substantially equal and the sensitivity ofpolarization shifter 114 will be substantially maximum.

The wave energy traveling from left to right, which emerges frompolarization means 104 and is incident on the left face of polarizationshifter 114, must inherently have the linear polarization P,, sincepolarization means 104 retains only wave energy with linear polarizationP within the resonant optical cavity. However, the wave energy travelingfrom right to left which emerges from the left face of polarizationshifter 114 may include a component of wave energy having a linearpolarization P which component has an amplitude determined by the totalphase delay between the ordinary and extraordinary portions of the waveenergy experienced during their round-trip passage through polarizationshifter 114. However, in the special case where this total phase delayhappens to be equal to an integral number of wave lengths of the waveenergy in the optical resonant cavity, the amplitude of the P linearpolarization component of the wave energy traveling from right to leftwhich energes from the left end of polarization shifter 114 will bezero.

Furthermore, this total phase delay, and hence the polarization shiftexperienced, depends upon the level of the signal applied topolarization shifter 114 from data bit stream source 118. In general,the amplitude of the P linearly polarized component in the wave energyreaching polarization means 104 from the right has a first given valuein response to the level of a data bit manifesting a binary ONE and asecond given value different from the first given value in response tothe level of a data bit manifesting a binary ZERO. By

choosing this latter level to be that required to provide a total phasedelay equal to an integral number of wave lengths of the wave energypassing through polarization shifter 114, the amplitude of P linearlypolarized wave energy incident on polarization means 104 from the rightcan be made equal to zero for a binary ZERO.

As is known in the art, mode-locking the wave energy within the opticalresonant cavity defined by reflectors and 102 is effected by applyingthe periodic signal from generator 116 to mode-locking element 112. Inparticular, a single, short duty cycle pulse of wave energy occursduring each cycle of the periodic signal. Since the average power of amode-locked laser is substantially the same as an equivalent CW laser,the peak amplitude of the mode-locked pulses of wave energy is increasedas an inverse function of the duty cycle of the pulses, as is known inthe art.

Because generator 116 is synchronized by data bit stream source 118, theduration of each data bit will just be equal to a single cycle of theperiodic signal frm generator 116. Thus, if, by way of example, each ofthe first and fourth of four successive data bits manifests a binary ONEand each of the second and third of these four successive data bitsmanifests a binary ZERO, the data bit stream output signal correspondingto these four successive bits from source 118, applied to polarizationshifter 114, will be that shown in the bottom graph of FIG. 2.

Assuming that level 200, manifesting a binary ZERO, corresponds with thespecial case, discussed above, in which the amplitude of the P linearlypolarized component of wave energy is zero, all the wave energytraveling from right to left incident on polarization means 104 willhave P linear polarization. Therefore, all of this wave energy passesthrough polarization means 104 and is retained within the opticalresonant cavity defined by reflectors 100 and 102. Under this condition,no output wave energy at all will be directed by polarization means 104out the bottom thereof as output wave energy. This condition isillustrated in the top graph of FIG. 2, wherein the circulating waveenergy in the optical resonant cavity, having the P linear polarization,has the relatively high amplitude A during the second and third data bitperiods when the data bit stream has level 200.

During the first and fourth data bit periods, when the data bit streamhas level 202, polarization shifter 114, in the manner described above,shifts the polarization of the wave energy traveling from right to leftthat emerges therefrom, the wave energy incident on polarization means104 contains a relatively small, but significant, component having Plinear polarization. However, the remainder of the wave energy travelingfrom right to left incident on polarization means 104 still has P linearpolarization. Therefore, during the first and fourth data bit period,when the data bit stream has level 202, P linearly polarized output waveenergy having amplitude B, as shown in the middle graph of FIG. 2, willbe directed by polarization means 104 out the bottom thereof. In thiscase, as shown in the top graph of F IG. 2, the P linearly polarizedcirculating wave energy retained in the optical resonant cavity willhave an amplitude A. While amplitude A is somewhat lower than amplitudeA, due to the fact that a portion of the wave energy in the resonantcavity is removed therefrom as output wave energy having amplitude B,still amplitude A is sufficiently high to maintain the synchronousgeneration of the mode-locked pulses of circulating wave energy. Hence,the concomitant pulses of output wave energy, manifesting the binaryvalue of each bit in the data stream, also occur synchronously.

A binary ZERO can, of course, be manifested by the presence of a pulseof output wave energy, rather than the absence thereof as is the case inFIG. 2, so long as the relative amplitudes of the respective pulses manifesting a binary ONE and a binary ZERO are different from each other.

In describing modulation means 110, in connection with FIG. 1, it wasassumed that the respective modelocking function and polarizationshifting function of the modulation means were accomplished by separatestructural elements, consisting of mode-locking element 112 andpolarization shifter 114. However, as shown in FIG. 1a, a singlestructural element may be employed for performing both the functions ofmodelocking and polarization shifting. In particular, modulation means110a of FIG. 1a comprises a single electrooptical crystal 120, which hasa dc. bias 122 of a given magnitude applied thereto. The signals fromperiodic signal source 116 and from data stream source 118 may becombined by such means as signal coupler 124, for example, and appliedin common to electro-optical crystal 120 through capacitance 126. Themagnitude of dc bias 122 is, preferably, set to that value which ensuresthat there will be an absence of output wave energy, having a linearpolarization P in response to the data bit stream manifesting level 200.

Referring now to FIG. 3, there is shown a first optical resonant cavitydefined by reflectors 300 and 302, and situated therewithin polarizationmeans 304, laser medium 306 (pumped by pump 308) and modulator 310(having respective signals from periodic signal generator 316 and datastream source 318 applied thereto).

Each of elements 300, 302, 304, 306, 308, 310, 316 and 318 of FIG. 3 aresimilar in structure to and correspond with each of respective elements100, 102, 104, 106, 108, 110, 116 and 118 of FIG. 1, described above. Inaddition, FIG. 3 includes reflector 330 which cooperates with reflector300 and polarization means 304 to form a second optical resonant cavity,and modulator 332 situated, as shown, between reflector 330 andpolarization means 304 within the second optical resonant cavity.Modulator 332 is identical in structure to modulator 310. The sameperiodic signal applied to the mode-locking element of modulator 310 byperiodic signal generator 316 is also applied to the modelocking elementof modulator 332. However, data bit stream source 318 applies a firstdata bit stream to the polarization shifter of modulator 310 and asecond data bit stream to the polarization shifter of modulator 332.Although the data bit stream applied to modulator 332 may be identicalto that applied to modulator 310, normally it represents different data.However, the respective first and second data bit streams appliedrespectively to each of the two modulators 310 and 332 are bothsynchronized by the synchronization input to data bit stream source 318from periodic signal generator 316.

Considering now the operation of FIG. 3, laser medium 306, in responseto being pumped by pump 308, generates wave energy at a givenwavelength. This wave energy, which, as will be shown, is unpolarized,can be considered to be composed of a first component having a firstlinear polarization P,, traveling from left to right, which passesthrough polarization means 304 and is then incident on modulator 310,and a second component having a linear polarization P traveling fromleft to right, which is incident on the left side of polarization means304 and is directed thereby out the top of polarization means 304 and isthen incident on modulator 332.

The P, linearly polarized wave energy incident on the left side ofmodulator 310, in the process of traveling from left to right throughmodulator 310, being reflected from reflector 302, and then travelingfrom right to left back through modulator 310, may have its modulationshifted by the polarization shifter of modulator 310 in accordance withthe binary value manifested by the data bit stream applied thereto.Therefore, the wave energy emerging from the left side of modulator 310and traveling from right to left incident on the right side ofpolarization means 304 may include a component having P linearlypolarized wave energy, in addition to the component having P, linearlypolarized wave energy. As described in connection with FIG. 1, the Plinearly polarized wave energy incident on the right side ofpolarization means 304 emerges from the bottom of thereof andconstitutes a first component of output wave ene gy, While the P,linearly polarized component passes through polarization means 304 and.is retained within the optic resonant cavity defined by reflector 300and 302.

In a similar manner, P linearly polarized wave energy incident on thebottom side of modulator 332 may have its polarization shifted by thepolarization shifter of modulator 332 during its passage from bottom totop through modulator 332, reflection from reflector 330 and then itstravel from top to bottom through modulator 332, so that the wave energyemerging from the bottom of modulator 332 may include a P, linearlypolarized component in addition to the P linearly polarized component,which travel from top to bottom and are incident on the top side ofpolarization means 304. In this case, the P, linearly polarizedcomponent will pass through polarization means 304 and emerge from thebottom thereof, constituting a second component of output wave energy.Therefore, the output wave energy emerging from the bottom ofpolarization means 304 includes two mutually orthogonal polarizationcomponents derived respectively by means ofmodulator 310 and modulator332 of the respective first and second optical resonant cavities. The Plinearly polarized component emerging from the bottom of modulator 332and incident on the top of polarization means 304 emerges from the leftside of polarization means 304 and is retained within the second opticalresonant cavity defined by reflector 300 and 330. v Thus, in thearrangement of FIG. 3, components-of wave energy having both P, and Plinearly polarization are returned to the common portion of the-firstand second optical resonant cavities situated between re-' flector 300and polarization means 304. This differs from the case in FIG. 1, whereonly wave energy having P, linear polarization is returned through theportion of the optical resonant cavity to the left of polarization means104. Therefore, in the arrangement of FIG. 3, the circulating waveenergy includes components of both P, and P polarization, and thus isunpolarized. This is opposed to the arrangement in FIG. 1 where thecirculating wave energy includes only P, linear polarization. Oneadvantage of this is that the practical problem of differential heatingof the laser medium, which causes birefringence which tends to detunethe cavity, is much less severe in the arrangement of FIG. 3 whereunpolarized wave energy passes through the laser medium than it is inthe case of FIG. 1 where polarized wave energy passes through themedium.

In FIG. 3, it is essential that the respective optical length s of thefirst and second optical resonant cavities be substantially equal toeach other. The reason for this is that the circulating mode-lockedpulse in the second optical resonant cavity must occur in phasecoincidence with the mode-locked resonant pulse in the first opticalresonant cavity, since the two optical resonant cavities share in commonreflector 300, laser medium 306 and the path between reflector 300 andpolarization means 304. Achieving this phase coincidence means that onlya single periodic signal can be applied to the mode-locking element ofrespective modulators 310 and 332. Since the mode-locking frequency ofthis periodic signal is a function of optic length, both the first andsecond optical resonant cavities must, therefore, have substantially thesame optical length to achieve the required phase coincidence of themodelocked pulses from the two cavities. Furthermore, slight-third-ordernon-linear effects in laser medium 306 provide sufficient cross-couplingto effect phase locking of the wave energy of the coincident modelockedpulses as they travel through laser medium 306.

FIG. 4 shows that the principles of FIG. 3 may be employed with ananalog data source. In particular, in the case of FIG. 4, mode-lockingis dispensed with, analog data source 418 replaces data bit streamsource 318 of FIG. 3, and modulators 410 and 432 each consist solely ofan electro-optic crystal responsive to separate respective analogsignals applied thereto from analog data source 418. In all otherrespects, the arrangement of FIG. 4 is equivalent to that of FIG. 3. Ofcourse, in the case of FIG. 4, the output of wave energy consists of twoorthogonally polarized components of continuous wave energy, each ofwhich is individually amplitude modulated in accordance with therespective separate analog signals from analog data source 418. Since nomode-locking takes place in the arrangement of FIG. 4, the first andsecond optical resonant cavities need not have substantially the sameoptical lengths. The requirement in the case of FIG. 4 is that theoptical length of the two cavities either be substantially equal to eachother or differ from each other by substantially an integral number ofwavelengths of the circulating wave energy.

We claim:

1. In an optical data transmission system of the type comprising threereflectors for reflecting light of certain wavelengths, an active lasingmedium effective when pumped and when located in an optical resonantcavity for generating light of at least one of said certain wavelengths,and polarization means; wherein said active lasing medium is locatedbetween a first of said reflectors and said polarization means, a secondof said reflectors is located with respect to said polarization meansand said first reflector to form a first optical resonant cavity for afirst component of said light having a first polarization which passesfrom said first reflector through said lasing medium and polarizationmeans toward said second reflector, a third of said reflectors islocated with respect to said polarization means and said first reflectorto form a second optical resonant cavity for a second component of saidlight having a second polarization in quadrature with said firstpolarization which passes from said first reflector through said lasingmedium and polarization means toward said third reflector; theimprovement therein, comprising:

modulation means including a signal-controlled polarization shifterlocated in said first cavity between said polarization means and saidsecond reflector for rotating the polarization of said light whichpasses therethrough by an amount determined in accordance with the levelof a control signal applied to said polarization shifter, wherein saidpolarization means is oriented with respect to said polarization shifterto eject as an output from said cavities that component of said lighthaving said second polarization which has passed from said secondreflector through said polarization shifter to said polarization means,and

wherein the length of each of said first and second cavities issubstantially equal to an integral number of half-wavelengths of thesame given one of said certain wavelengths.

2. The output data transmission system defined in claim 1, furtherincluding an analog signal source coupled to said polarization shifterto provide said control signal thereto for rotating the polarization ofsaid light passing therethrough in accordance with the instantaneouslevel of said analog signal.

3. The optical data transmission system defined in claim 2, furtherincluding a second analog signal source, a second signal-controlledpolarization shifter having said second signal source coupled thereto toprovide said control signal thereto, said second polarization shifterbeing located in said second cavity between said polarization means andsaid third reflector for rotating the polarization of said light whichpasses therethrough by an amount determined in accordance with theinstantaneous level of the analog signal applied thereto from saidsecond analog signal source, and

wherein said polarization means is oriented with respect to said secondpolarization shifter to eject as a second output from said cavities thatcomponent of said light having said first polarization which has passedfrom said third reflector through said second polarization shifter tosaid polarization means.

4. The optical data transmission system defined in claim 1, wherein saidmodulation means further includes mode-locking means, and wherein systemfurther includes a periodic signal generator coupled to provide an inputto said mode-locking means, said generator generating a frequencysubstantially equal to an integral multiple of the speed of lightdivided by twice the length of said first cavity to thereby effectmodelocking with said generated frequency, and a data bit stream sourcesynchronized with said generated frequency, said data bit stream sourcebeing coupled to said polarization shifter to provide said controlsignal thereto.

5. The optical data transmission system defined in claim 4, wherein saidfirst and second cavities have substantially the same length, andwherein said system further includes a second data bit stream sourcesynchronized with said generated frequency, second modulation meanscomprising a second signal-controlled polarization shifter having saidsecond data bit stream source coupled thereto to provide said controlsignal wherein said polarization means is oriented with respect to saidsecond polarization shifter to eject as a second output from saidcavities that component of said light having said first polarizationwhich has passed from said third reflector through said secondpolarization shifter to said polarization means.

1. In an optical data transmission system of the type comprising threereflectors for reflecting light of certain wavelengths, an active lasingmedium effective when pumped and when located in an optical resonantcavity for generating light of at least one of said certain wavelengths,and polarization means; wherein said active lasing medium is locatedbetween a first of said reflectors and said polarization means, a secondof said reflectors is located with respect to said polarization meansand said first reflector to form a first optical resonant cavity for afirst component of said light having a first polarization which passesfrom said first reflector through said lasing medium and polarizationmeans toward said second reflector, a third of said reflectors islocated with respect to said polarization means and said first reflectorto form a second optical resonant cavity for a second component of saidlight having a second polarization in quadrature with said firstpolarization which passes from said first reflector through said lasingmedium and polarization means toward said third reflector; theimprovement therein, comprising: modulation means including asignal-controlled polarization shifter located in said first cavitybetween said polarization means and said second reflector for rotatingthe polarization of said light which passes therethrough by an amountdetermined in accordance with the level of a control signal applied tosaid polarization shifter, wherein said polarization means is orientedwith respect to said polarization shifter to eject as an output fromsaid cavities that component of said light having said secondpolarization which has passed from said second reflector through saidpolarization shifter to said polarization means, and wherein the lengthof each of said first and second cavities is substantially equal to anintegral number of half-wavelengths of the same given one of saidcertain wavelengths.
 2. The output data transmission system defined inclaim 1, further including an analog signal source coupled to saidpolarization shifter to provide said control signal thereto for rotatingthe polarization of said light passing therethrough in accordance withthe instantaneous level of said analog signal.
 3. The Optical datatransmission system defined in claim 2, further including a secondanalog signal source, a second signal-controlled polarization shifterhaving said second signal source coupled thereto to provide said controlsignal thereto, said second polarization shifter being located in saidsecond cavity between said polarization means and said third reflectorfor rotating the polarization of said light which passes therethrough byan amount determined in accordance with the instantaneous level of theanalog signal applied thereto from said second analog signal source, andwherein said polarization means is oriented with respect to said secondpolarization shifter to eject as a second output from said cavities thatcomponent of said light having said first polarization which has passedfrom said third reflector through said second polarization shifter tosaid polarization means.
 4. The optical data transmission system definedin claim 1, wherein said modulation means further includes mode-lockingmeans, and wherein system further includes a periodic signal generatorcoupled to provide an input to said mode-locking means, said generatorgenerating a frequency substantially equal to an integral multiple ofthe speed of light divided by twice the length of said first cavity tothereby effect mode-locking with said generated frequency, and a databit stream source synchronized with said generated frequency, said databit stream source being coupled to said polarization shifter to providesaid control signal thereto.
 5. The optical data transmission systemdefined in claim 4, wherein said first and second cavities havesubstantially the same length, and wherein said system further includesa second data bit stream source synchronized with said generatedfrequency, second modulation means comprising a second signal-controlledpolarization shifter having said second data bit stream source coupledthereto to provide said control signal thereto and a second mode-lockingmeans having said generated frequency applied thereto, said secondmodulation means being located in said second cavity between saidpolarization means and said third reflector for rotating thepolarization of said light which passes through said second polarizationshifter by an amount determined by said data bit stream applied theretofrom said second data bit stream source, and wherein said polarizationmeans is oriented with respect to said second polarization shifter toeject as a second output from said cavities that component of said lighthaving said first polarization which has passed from said thirdreflector through said second polarization shifter to said polarizationmeans.